As to present-day dry toners, there is an increased demand for improvement in image quality with the use of the digital copying system, which necessitates controlling toner particle diameters, in other words which necessitates obtaining toners with a sharp particle size distribution by reducing the proportions of fine powder and coarse powder, and thus there is an increased demand for development of toner producing apparatuses for obtaining toners with a desired particle size distribution.
Conventionally, for a pulverizing apparatus incorporated in a pulverizing and coarse powder classifying apparatus for toner, one or two mechanical pulverizers are used wherein each of the mechanical pulverizers includes a rotor having concavities and convexities on its surface, and a stator whose concave-convex surface is stationarily placed on a circumferential inner side of the rotor, in which a toner raw material is pulverized by means of a vortex generated between the rotor and the stator by rotating the rotor at high speed. Further, fine powder is separated from the pulverized toner raw material by two classifying units joined to the pulverizer/pulverizers, and a toner as a product is thus obtained.
FIG. 1 shows an example of a flow of steps in a conventional toner production process. For example, a pulverizing and coarse powder classifying step employs closed-circuit pulverization, in which a raw material introduced through a raw material supply pipe 1 is pulverized by a pulverizing unit 2, then temporarily collected by a cyclone 4, and introduced to a coarse powder classifying unit 5 where the pulverized raw material is classified into pulverized material and fine powder. Coarse powder is returned to the pulverizing unit 2 via a pipe 3 so as to be pulverized again. Fine powder generated in the pulverizing step is collected by a cyclone 7 via a path 6 and subsequently supplied to a fine powder classifying step.
Powders supplied to the classifying unit in the fine powder classifying step include powder of the raw material, and toner with a wide range of particle diameters undergoing a pulverizing process, that has circulated between the pulverizing unit and the classifying unit. Therefore, the powders are of broad particle size distribution, which necessitates lowering the collection rate of a product to attain a desired particle size, and there is an increased amount of toner returned to a mechanical pulverizer that performs closed-circuit pulverization, which necessitates operation with a very large load. In the fine powder classifying step, the toner is further classified by a fine powder classifier 8, and a product is collected in a product collector 9.
Fine powder separated from the product is temporarily collected by a cyclone 11 via a path 10, then introduced to a fine powder classifier 14 via a path 12 so as to be classified again, and coarse particles are returned to the fine powder classifier 8 via a path 13. Meanwhile, fine powder is collected by a cyclone 16 via a path 15, then recovered as a fine powder 17. These fine powder classifiers 8 and 14 can be suitably selected from classifiers for single classification and classifiers for two-staged classification, according to the processing capability.
As described above, since the proportions of coarse powder and fine powder are large in a conventional toner producing method, images obtained using a developer thus produced are unstable in charge amount and variable in density. In other words, excessively pulverized toner, which affects the charge amount of the toner, causes background smear, and insufficiently pulverized toner causes transfer failure that leads to image defects and reduction in image quality. Moreover, in production of toner, an excessive load is applied to a classifier, so that classification efficiency is poor, and pulverizing energy efficiency in pulverization is also poor, which is problematic.
Meanwhile, in a pulverizing method using a jet pulverizer, the number-based percentage of unwanted fine powder is 15% to 50%, which is fairly large. Thus, the fine powder is liable to be mixed into the toner product; further, production efficiency becomes poor because the fine powder needs to be removed, and additional energy is required to reuse the removed fine powder.
Patent Literature 1 discloses a classifier provided with a dispersion section and a classification section, and the like, and Patent Literature 2 discloses a classifier in which a secondary airflow vane is provided on the upper circumference of a raw material supply pipe. However, both the classifiers in Patent Literatures 1 and 2 lack a function for enhancing classification accuracy by increasing the swirling velocity in the classifiers; also, classification takes place only once in the classifiers owing to the structures thereof, which causes reduction in the classification accuracy, and coarse particles are mixed into a pulverized material in classification of coarse powder, whereas fine powder is mixed into a product in classification of fine powder, which causes accuracy reduction. Hence, the conventional classifiers give insufficient pulverizing performance owing to their poor pulverizing capability and power consumption. Further, the particle size and particle size distribution of toner are important in view of image quality; therefore, there is a problem in which toners produced using the classifiers have negative effects on the distribution of charge amount, etc.    [Patent Literature 1] Japanese Utility Model Application Laid-Open (JP-U) No. 58-013956    [Patent Literature 2] Japanese Patent (JP-B) No. 2766790